The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with split-top insertable radio frequency coils for magnetic resonance imaging of the head and neck and will be described with particular reference thereto. It is to be appreciated, however, that the present invention will also find application in other multiple coil techniques, spectroscopy, phased array coils, imaging for other than medical diagnostic purposes, and the like.
Conventionally, magnetic resonance imaging systems generate a strong, uniform static magnetic field B.sub.0 in a free space or bore of a magnet. This main magnetic field polarizes the nuclear spin system of an object in the bore to be imaged. The polarized object then possess a macroscopic magnetic moment vector pointing in the direction of the main magnetic field. In a superconducting main magnet assembly, annular magnets generate the static magnetic field B.sub.0, along a longitudinal or z-axis of the cylindrical bore.
To generate a magnetic resonance signal, the polarized spin system is excited by applying a radio frequency field B.sub.1, perpendicular to the z-axis. Typically, a radio frequency coil for generating the radio frequency field is mounted inside the bore surrounding the sample or patient. In a transmission mode, the radio frequency coil is pulsed to tip the magnetization of the polarized sample away from the z-axis. As the magnetization precesses around the z-axis back toward alignment, the precessing magnetic moment generates a magnetic resonance signal which is received by the radio frequency coil in a reception mode.
For imaging, a magnetic field gradient coil is pulsed for spatially encoding the magnetization of the sample. Typically, the gradient magnetic field pulses include gradient pulses pointing in the z-direction but changing in magnitude linearly in the x, y, and z-directions, generally denoted G.sub.x, G.sub.y, and G.sub.z, respectively. The gradient magnetic fields are typically produced by a gradient coil which is located inside the bore of the magnet and outside of the radio frequency coil.
Conventionally, when imaging the torso, a whole body radio frequency coil is used in both transmit and receive modes. By distinction, when imaging the head, neck, shoulders, or an extremity, the whole body coil is often used in the transmission mode to generate the uniform B.sub.1 excitation field and a local coil is used in the receive mode. Placing the local coil close to the imaged region improves the signal-to-noise ratio and the resolution. In some procedures, local coils are used for both transmission and reception. One drawback to local coils it that they tended to be relatively small. The whole body coils are typically used for elongated regions, such as the spine. One technique for adapting surface coils for imaging an elongated region is illustrated in U.S. Pat. No. 4,825,162 of Roemer, in which a series of surface coils are lapped to construct a phased array.
Other radio frequency coil designs include a multi-modal coil known as the "birdcage" coil. See, for example, U.S. Pat. No. 4,692,705 of Hayes. Typically, a birdcage coil has a pair of end rings which are bridged by a plurality of straight segments or legs. In a primary mode, currents in the rings and legs are sinusoidally distributed which results in improved homogeneity along the axis of the coil. Homogeneity along the axis perpendicular to the coil axis can be improved to a certain extent by increasing the number of legs in the coil. Typically, a symmetric birdcage coil has eight-fold symmetry. Such a symmetric birdcage coil with N legs (where N is an even integer) exhibits N/2 mode pairs. The first (N/2)-1 mode pairs are degenerate, while the last mode pair is non-degenerate. The primary mode of such an eight-fold symmetric birdcage coil has two linear modes which are orthogonal to each other. The signals from these two orthogonal or quadrature modes, when combined, provide an increased signal-to-noise on the order of 40%. The simplest driven current pattern or standing waves defined by superpositions of degenerate eigenfunctions. For a low-pass birdcage of N meshes driven at is lowest non-zero eigenfrequency, the current in the n-th mesh is given by sin (2 .pi.n /N+.phi.). The phase angle .phi. determines the polarization plane of the resulting B.sub.1 radio frequency field and can be varied continuously by suitable application of driving voltages. The alignment and isolation of the two linear modes of a birdcage coil are susceptible to sample geometry. That is, the sample dominates the mode alignment and isolation between the two linear modes.
Birdcage coils, like other magnetic field coils, undergo mutual inductive coupling when positioned adjacent each other. As the coils approach each other, the mutual inductive coupling tends to increase until a "critical overlap" is reached. At the critical overlap, the mutual inductance drops to a minimum. As the coils are moved towards a complete coincidence from the critical overlap, the mutual inductive coupling again increases. See, "Optimized Birdcage Resonators For Simultaneous MRI of the Head and Neck", Leussler, SMRM, 12th Annual Meeting, Book of Abstracts, page 1349(1993).
In one multiple coil birdcage design, two birdcage coils have been overlapped to a point of minimum mutual inductance. A symmetric coil is used to image the head and an asymmetric coil was used to image the neck. Capacitive elements are added to provide the necessary phase shifting through the leg sections of unequal length. The coils are mounted in a rigid frame for optimum symmetry and a fixed geometric position. The coils are isolated from each other by the critical overlapping as well as by the addition of neutralization capacitors. See for example, U.S. Pat. No. 4,769,605 issued Sep. 6, 1988 to Timothy R. Fox.
Building an asymmetric coil design is fairly complicated and time-consuming. The phase shift from one section to another in the birdcage coil needs to be maintained for optimum coil performance. The critical overlapping between the two birdcage coils reduces the mutual coupling between the coils to a certain extent. Introducing different coil samples into the two birdcage coils alters the alignment of their linear modes and the mode isolation in either of the coils will change. This change, in turn, affects the symmetry and therefore the mutual coupling between the coils. The greater the mutual coupling between the coils, the larger the noise correlation between the coils and therefore the lower the combined signal-to-noise ratio. Electrical optimization of such a coil design is very complex. The tuning, matching, and isolation process is iterative and thus time-consuming. More specifically, the two linear modes of each birdcage coil need to be tuned, matched, and aligned to their respective coupling points on the coil and isolated from one another and from the two linear modes of the neck coils. Such a complex and iterative tuning, matching, and isolation process is not readily amenable to mass production.
"Novel Two Channel Volume Array Design For Angiography of the Head and Neck", Reykowski, et al., SMR 2nd Annual Meeting, Book of Abstracts, pp. 216 (1994), discloses a birdcage coil in combination with two volumetric Helmholtz coils arranged such that the B.sub.1 fields of the two Helmholtz coils are diagonal and perpendicular to one another. The two quadrature combined outputs, one from the birdcage coil and one from the Helmholtz coils are interfaced to two channels of the system. By orienting the two Helmholtz coils such that their B.sub.l fields are orthogonal, coupling is reduced and the noise correlation therebetween held to a minimum. However, when these two volume coils are overlapped with a quadrature head coil, they experience the same difficulties discussed above in conjunction with the multiple birdcage coils. That is, when different sampled geometries are introduced, the isolation between the individual volume Helmholtz coils and the head coil change, causing a change in isolation, resulting in an increased noise correlation between all coils and a lower combined signal-to-noise ratio. Manufacturability of the coil is again complex and time-consuming.
The problem of coil interaction generally exists whenever two or more volume coils of different geometries are used. Electrical optimization of these coil designs is often complicated and iterative, hence time-consuming. Different sample geometries introduced into the designs alter coil-to-coil isolation, resulting in different noise correlations between the coils from one patient to another.
"Head and Neck Vascular Array Coil For MRI", Srinivasan, et al., SMR 2nd Annual Meeting, Book of Abstracts, pp. 1107(1994) and the applicants' co-pending earlier filed related U.S. patent application Ser. No. 08/343,635, filed Nov. 22, 1994 disclose a combination birdcage coil and quadrature volume coil pair. In the described coil design, the coils maintained different current distributions with preferred mode orientations independent of one another. The coil consists of a birdcage coil and a quadrature volume coil pair. The quadrature volume coil pair consists of at least two surface coils of the distributed type, that maintain preferred mode orientations with respect to one another at all times. The birdcage coil maintains an eight-fold symmetry; whereas, the surface coil maintains a two-fold symmetry. After a nominal overlap is achieved between the coils of this design, only one iteration of tuning is required to retune all coils to the magnetic resonance frequency.
The present invention provides a new and improved radio frequency coil which overcomes the above-referenced problems and others.